As one of measures to cope with noises, a silencer is mounted on an exhaust pipe of a motorcycle thus realizing the reduction of exhaust noises discharged from an internal combustion engine (for example, see Japanese Patent Application No. 2007-56714, FIG. 1 and FIG. 2).
FIG. 2 of patent document 1 shows the cross-sectional structure of a silencer (15) (numeral with a parenthesis indicating a symbol described in patent document 1, and same indication being applicable hereinafter). That is, the silencer (15) is configured such that a semispherical front cap portion (18) is mounted on a front end of an outer sleeve portion (17), a rear cap portion (19) is mounted on a rear end of the outer sleeve portion (17), and a sound absorbing material (20) is filled in the inside of the outer sleeve portion (17).
Acoustic energy of an exhaust gas which flows in an inner sleeve portion (16) is attenuated due to sudden expansion of the exhaust gas squeezed by sound absorbing holes (16c), and the sound absorbing material (20) absorbs sounds so that a silencing effect is acquired.
In the silencer (15) having such structure, by increasing a length of the outer sleeve portion (17) or by increasing an outer diameter of the outer sleeve portion (17), a volume of an expansion chamber can be increased and hence, silencing performance can be enhanced as a result.
As shown in FIG. 1 of patent document 1, the silencer (15) is arranged above a rear wheel (6). When the length of the outer sleeve portion (17) is increased aiming at the enhancement of silencing performance, the rear cap portion (19) projects rearward in the longitudinal direction of a vehicle from a rear fender (not indicated by a symbol).
When a length of the silencer (15) is increased so that the silencer (15) extends rearward in the longitudinal direction of the vehicle, a center of gravity of the silencer (15) is shifted toward a rear side of the vehicle thus influencing the concentration of a mass which a motorcycle is required to possess.
Amidst the demand for enhancement of performance of a silencer as one of measures to cope with an environmental problem, a silencer which does not influence the concentration of a mass even when a length of the silencer is increased is requested.